How is wind shear defined?

Wind shear is defined as the rate of change of wind velocity per unit of distance. This phenomenon occurs when there is a difference in wind speed or direction at different altitudes or over a horizontal distance. Understanding wind shear is crucial in aviation because it impacts aircraft performance and safety, especially during takeoff and landing phases.

While the other options refer to different meteorological concepts, they do not describe wind shear correctly. The change in temperature with altitude relates to the atmosphere's thermal structure; the difference in wind speed between two altitudes may describe a characteristic of wind shear but does not encompass its definition as it does not address the rate of change aspect. The variation of humidity across a distance pertains to moisture content in the air, which is unrelated to wind patterns. Thus, the definition that emphasizes the rate of change of wind velocity is the most accurate and aligns directly with the concept of wind shear.